Hydrocolloids, particularly polysaccharide gums, are widely employed in the food industry to stabilize disperse foods such as emulsions and foams (e.g. ice cream, whipped toppings, beer, salad dressings, sausages, etc.) see S. C. Sharma, Gums and Hydrocolloids in Oil Water Emulsions, J. Food Tech. January 1981, 59-67. These foods are polydisperse colloidal systems and are generally thermodynamically inherently unstable, i.e. in the absence of specific stabilizing agents such as polysaccharides the physical state of the processed food rapidly deteriorates. For example, ice creams develop ice crystals and become grainy, salad dressings separate, and sausages both exude oil and exhibit syneresis or weeping. The precise role of the polysaccharide is not well established in complex foods in which they are used; however, their ability in model systems to modify both the bulk properties of the continuous aqueous phase and the interfacial behavior of other components, particularly proteins, is considered indicative of their beneficial effects.
Levan or anhydrofructosylfructoside is a linear polymer of adjacent fructose residues covalently bonded together in the beta conformation between the 2- and 6- carbon atoms (E. A. Dawes et al., Biochem. J. 98: 804-812, 1966.) This polymer can be formed by the microbial fermentation of sucrose through a transfructosylation reaction catalyzed by the enzyme levan sucrase (E.C. 2.4.1.10). Several strains of bacteria have been described which elaborate a functional levan sucrase. These include strains of Actinomyces viscosus, Aerobacter levanicum, Acetobacter suboxydans, Bacillus licheniformis, Bacillus macerans, Bacillus subtilus, Gluconobacter oxydans, Streptococcus salivarius, Streptococcus mutans, Leuconostoc mesenteroides, Erwinia herbicola and Zymomonas mobilis. Levan can also be formed in vitro by the action of levan sucrase on either sucrose or raffinose which functions as a fructosyl donor and an aldose molecule which acts as the fructosyl acceptor.
Levan has been used as an immunologic suppressor (Experientia 34: 1362-1363, 1978 and J. Pathol. 125: 103, 1978) and an immunogenic hapten conjugate (J. Dent. Res. 55: 129, 1976 and Moreno, U.S. Patent No. 4,260,602). UK Patent Application GB No. 2,046,757-A published Nov. 19, 1980 describes a low cariogenic food sweetener prepared by subjecting a substrate solution containing aldose and sucrose or raffinose to the activity of levan sucrase, but the desired product is a trisaccharide and not a high molecular weight levan, which has not heretofore been to provide functional properties to food products.
Traditionally, small quantities of levan have been produced by the bacterial fermentation of sucrose, usually by a strain of Actinomyces viscosus or Aerobacter levanicum. Levan produced by the in vitro activity of levan sucrase has been reported by T. Tanaka et al. in J. Biochem. 87 (1): 297-303 (1980). The applications of recombinant DNA technology have been applied in the vitro genetic transfer of the sacB gene from Bacillus subtilis into a strain of Escherichia coli (J. Bacteriol. 153: 1424-31, 1983.) These processes, however, do not represent efficient means for the production of levan. In particular, the in vitro enzymatic synthesis of levan from sucrose would appear to be difficult in terms of producing a high molecular weight molecule, as evidenced by UK Patent Application GB No. 2,046,757-A.
The in vivo production of levan has also been described, e.g. in Jeanes et al. U.S. Pat. No. 2,673,828; Gaffar et al. U.S. Pat. No. 3,879,545; and Ayerbe et al. U.S. Pat. No. 4,399,221, the contents of which are incorporated by reference herein. Such methods have generally employed aerobic fermentation techniques in the presence of relatively small amounts of sucrose (i.e., less than about 2%) in the feedstock and have achieved relatively low yields.